1. Field of the Invention
The invention relates to a light emitting device having, on a substrate with insulating surfaces, a circuit comprising thin film transistors (referred below to as TFT) and a light emitting element between a pair of electrodes to be formed with a light emitting layer.
2. Related Art
Since TFT can be formed on a transparent substrate, application and development thereof on active matrix type picture display units have been positively promoted Because high mobility can be obtained with TFT, which makes use of a crystalline semiconductor film (typically, crystalline silicon film), picture display of high definition can be realized by integrating function circuits on the same substrate. In recent years, attention has been paid to a technique, in which TFT is constituted by means of a semiconductor thin film (having a thickness in the order of several to several hundreds nm) formed on a substrate having insulating surfaces. TFT has been widely applied on electron devices such as IC and electro-optical devices, and in particular, development thereof as a switching element for picture display units has been hastened.
Various applications making use of such picture display units are expected and in particular, attention has been paid to application on portable equipments. Among other things, a light emitting element has no need of back light as used in conventional liquid crystal displays. Accordingly, a light emitting element has a great advantage in that it can be fabricated to be extremely thin and lightweight.
A light emitting element is one of self-light emission type, in which a light emitting layer itself emits light upon application of electric field. A light emitting mechanism is said to consist of interposing a light emitting layer between electrodes to apply voltage thereon whereby electrons injected from a negative electrode and positive holes injected from a positive electrode rejoin at an emission center in the light emitting layer to form molecular excitons, which discharge energy to cause light emission when returning to a normal state.
With an active matrix type light emitting device having a light emitting element, a TFT is formed on a substrate and an interlayer insulating film is formed on the TFT. In addition, such interlayer insulating film can be formed by using an inorganic material containing silicon such as silicon oxide and silicon nitride, or an organic material such as organic resin materials, for example, polyimide, polyamide and acrylic resins.
A light emitting device means one comprising a positive electrode, a negative electrode, and a light emitting element constructed to have a layer, which is interposed between them and comprising a material, by which light emission through fluorescence or phosphorescence is obtained. In addition, a light emitting element referred to here and making use of an organic compound as a material for light emission is also called OLED (Organic Light Emitting Device).
In the case of flattening a substrate surface with an interlayer insulating film, formation of an interlayer insulating film by means of the spinning coating method is suitable. In performing a spinning coating, a solvent with the above silicon as a main component is used as an inorganic material. An interlayer insulating film formed by using a solvent with silicon as a main component possesses a property making oxygen or water impermeable therethrough, and the property exhibits itself when baked at sufficiently high temperature. Also, the film has a disadvantage that crack is generated when the film is increased in thickness.
Meanwhile, an interlayer insulating film formed by an organic material has an advantageous feature that while making oxygen permeable therethrough and making water permeable or absorbing water, it can be formed to have a thickness of 1 μm or more and its surface can be flattened under no influence from irregularities on a substrate. Also, the baking temperature is around 300° C. at the time of formation, and so the film can be also formed a substrate surface comprising an organic resin.
However, an organic material suited to the interlayer insulating film possesses a property making water vapor permeable therethrough and liable to absorb moisture. On the other hand, a light emitting layer, no matter whether it is low molecular or high molecular, has a disadvantage that it is extremely affected by oxygen and moisture to be deteriorated immediately. Further, alkali metal or alkaline earth metal is used for a positive electrode or a negative electrode of a light emitting element, and these are liable to be oxidized by oxygen. That is, moisture becomes responsible for deterioration of a light emitting element and failure such as dark spots or the like.
Silicon oxide, silicon nitride, or the like formed by the plasma CVD technique involves a need of heating a substrate at 300° C. or higher, and so is not suited to forming such silicon oxide, silicon nitride or the like on an organic resin film with no heat-resistance.